The effect of fluorine doping on the electrochemical performance of LiFePO 4 /C cathode material is investigated. The stoichiometric proportion of LiFe(PO 4 ) 1 −x F 3x /C (x=0.01, 0.05, 0.1, 0.2) materials was synthesized by a solid-state carbothermal reduction route at 650°C using NH 4 F as dopant. X-ray diffraction, scanning electron microscope, energy-dispersive X-ray, and X-ray photoelectron spectroscopy analyses demonstrate that fluorine can be incorporated into LiFePO 4 /C without altering the olivine structure, but slightly changing the lattice parameters and having little effect on the particle sizes. However, heavy fluorine doping can bring in impurities. Fluorine doping in LiFePO 4 /C results in good reversible capacity and rate capability. LiFe(PO 4 ) 0.95 F 0.15 /C exhibits highest initial capacity and best rate performance. Its discharge capacities at 0.1 and 5 C rates are 156.1 and 119.1 mAh g −1 , respectively. LiFe(PO 4 ) 0.95 F 0.15 /C also presents an obviously better cycle life than the other samples. We attribute the improvement of the electrochemical performance to the smaller charge transfer resistance (R ct ) and influence of fluorine on the PO 4 3− polyanion in LiFePO 4 /C.
LiFe0.9V0.1(PO4)0.95F0.15/C was prepared via solid-state carbothermal reaction (CTR). F and V codoping did not alter the olivine structure of LiFePO4 but reduced the particle size and improved the Li+ diffusion coefficient. The cells based on this material showed higher discharge capacity, working voltage, rate capability, and better cyclic performance than that of undoped and F-doped materials.
Melt shear flow behavior and melt shear viscosity are important characterization of processing properties for polymeric materials. The effects of microencapsulated red phosphorus (MRP) content and experimental conditions on the melt shear flow behavior of the polypropylene (PP) composites were investigated using a torque rheometer. It was found that the melt shear viscosity, the torque, and energy consumption at balance state of the composites increased nonlinearly with increasing MRP weight fraction while decreased slightly with increasing temperature, the maximum torque increased with increasing the roller speed, and the dependence of the melt shear viscosity and the energy consumption at balance state on temperature roughly obeyed the Arrhenius equation. These findings would provide useful data for processing these flame-retardant PP composites.
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